The invention relates generally to the field of polymer additives and specifically to high concentration pelletized additive concentrates, or polymer stabilization agent or blends, used in various polymerization processes to enhance polymer stability.
Polymer additives and additive blends are typically used to protect polymers from thermo-oxidative degradation, to provide long term resistance to light or heat, to neutralize residual catalyst and to enhance performance properties of the finished product. Polymer additives typically come in powder, granule, or pellet form. These additives can be routinely added to the polymer during post reactor extrusion operations. Numerous techniques may be employed to introduce the additives to the polymer stream. In solution, suspension or slurry phase polymerization processes, additives and additive blends are frequently added to a liquid before being introduced to the post-reactor polymer-liquid slurry. Alternatively, the additives can be added to the final melt stream of polymer via a side arm extruder or other device which can melt the additive and introduce them to the polymer stream. In this case, there will typically be further mixing via an extruder or other mixing device and pumping of the polymer/additive mixture through a die for pelletizing the final polymer. In other polymerization processes such as a gas phase reactor, the polymer exits the reactor as a powdered “reactor granule.” In this case, additives can be added to the polymer in several different ways. The additives can be added to the solid “reactor granule” powder stream. This can be packaged off as a final saleable product or it can be further fed to an extruder or other melting device in order to mix and homogenize the polymer and disperse the additives into the molten polymer. When additives are added to the solid “reactor granule” powder stream, the additives can be introduced at this stage via their neat forms, typically powders, or via a concentrate or masterbatch form. This mixture is subsequently pumped through a die for pelletization. Alternatively, in this type of process, the additives can be introduced via a side arm extruder. The side arm extruder melts the additives and feeds them into a molten polymer stream where they are further mixed into the final polymers and pelletized. In all of these techniques, the addition of the additives in powder form can be difficult to handle and feed, and in the case of some additives, they pose a potential health, fire, and explosion risk. If the polymer system requires the addition of several components, the additives must be either pre-blended, or the use of more than one feeder is required. When a side-arm extruder is used, it is not common to feed the powdered additives directly for numerous reasons. In addition to the above mentioned issues with handling and feeding the additives in powder form to the side-arm extruder, the melting and viscosity behaviour of the additives and the additive mixtures are typically not suitable for direct addition via this method. As a result, the powdered additives can be made into a fairly low concentration masterbatch. This type of masterbatch typically is made by extruding a low concentration of additives with a polymer carrier resin that is similar and compatible with the main polymer being produced in the polymerization process. As a result, this masterbatch can be easily fed via a side-arm extruder.
Preparation of non-dusting pellet forms of additive blends solves many of these problems. U.S. Pat. No. 5,240,642 entitled “Process for Obtaining Granular Forms of Additives for Organic Polymers” describes a process for making low-dust granules of an additive blend including a phenol antioxidant and an acid neutralizer processed in the amorphous or molten state including using an extruder.
U.S. Pat. No. 5,844,042 entitled “Process for Obtaining Granular Forms of Additives for Organic Polymers” describes granular forms of additive blends prepared by forcing the blend through a die to form strands and then cutting said strands to form pellets.
U.S. Pat. No. 5,597,857, entitled “Low-Dust Granules of Plastic Additives” describes additive pellets comprising 10-100% calcium stearates.
U.S. Pat. No. 6,740,694B2 entitled “Preparation of Low-Dust Stabilizers” describes using a sub-cooled melt of an additive as a carrier liquid for other additives and as well as amorphous versions of stabilizers.
U.S. Pat. No. 6,515,052 entitled “Granular Polymer Additives and Their Preparation” describes using a solvent in a compaction process to improve the yield and quality of a compacted additive blend including a phosphite.
U.S. Pat. No. 6,800,228 entitled “Sterically Hindered Phenol Antioxidant Granules Having Balance Hardness” describes using a solvent for the preparation of compacted additive blends which including a phenol.
The inventions described above provide for low-dusting forms of additive blends that can be more conveniently and accurately fed to post reactor extrusion operations for addition to a polymer when added directly to a polymer stream that is in the solid phase and premixed or fed simultaneously with the polymer stream into an extruder or other melting device whereby the polymer is melted and the additives are then blended into molten polymer. When the additives require addition via a side-arm extruder and fed directly to a molten polymer stream, the above described additive blends are not used. In this case, masterbatches or concentrates of additives or additive blends in a compatible polymer carrier are used. Masterbatches have the benefit of low friability of the pellet, they can be air conveyed, fed, and extruded using conventional equipment and methods by a side arm extruder.
The preparation of masterbatches is well known in the art. Masterbatches simplify the addition of at least one component to the polymer blend. For economic reasons it is desirable to prepare masterbatches with high levels of additives, and minimize the use of the compatible polymer carrier. This minimizes the amount of masterbatch required to achieve a desired effect.
The preparation of high levels of mineral filled masterbatches is well known in the art. U.S. Pat. No. 6,713,545 B2 entitled “Universal Masterbatch” describes a masterbatch of up to 85% filler, plus a viscosity modifier in a universal SBS carrier. A difficulty in the preparation of masterbatches of high filler concentration is wetting out, mixing and dispersing the filler while maintaining an adequately low viscosity to be able to process the masterbatch. The addition of high levels of filler can greatly increase the viscosity of the masterbatch.
U.S. Pat. No. 6,255,395 B1 to Klosiewicz entitled “Masterbatches Having High Levels of Resin” describes incorporating high levels of hydrocarbon resins into a polymer carrier. The resin preferably has a softening point near or above the softening point of the carrier polymer and has a sufficient viscosity to allow an extruder to put work into the mixture. Preparation of the masterbatches is accomplished above the softening point of the resin.
Many polymer additives, when heated to typical masterbatch processing temperatures, pass through a crystalline melting point or an amorphous phase transition to form low viscosity fluids. Such low viscosity fluids can be difficult to incorporate into a polymer carrier at high levels. Poorly incorporated additive can migrate out of the finished masterbatch pellet. This can cause dusting, stickiness and or agglomeration of the masterbatch pellets. Furthermore the low viscosity additive can substantially decrease the viscosity of the carrier-additive blend, causing difficulties in the pelletization process. For these reasons, masterbatches of polymer additives, with melting points near or below typical masterbatch processing temperatures, are prepared at only low to medium additive levels. It would therefore be advantageous to prepare more economical highly-loaded additive masterbatches of these additives.